Field of the Invention
The disclosure relates to a fixing device that is mounted in an image forming apparatus such as a copying machine or a printer and that fixes an unfixed image formed on a recording material to the recording material.
Description of the Related Art
As a fixing device mounted in an electro photographic copying machine or printer, a fixing device of a film heating type is known. The fixing device of the film heating type includes a tubular film, a heater that is provided in the internal space of the film and that is in contact with the inner surface of the film, and a pressure roller that forms a fixing nip portion together with the heater with the film there between. The heater is held by a resin heater holder provided in the internal space of the film. The heater holder is reinforced by a metal reinforcing member (stay) provided in the internal space of the film.
A through hole is provided in a part of a heater holding seat surface of the heater holder. A temperature detection element (sensor or the like) that detects the temperature of the heater is disposed in the space between the heater holder and the reinforcing member. The temperature detection element is inserted into the through hole of the heater holder. The heater is controlled according to the detected temperature of the temperature detection element. In the space between the heater holder and the reinforcing member, a safety element (thermal switch, temperature fuse, or the like) for interrupting power supply to the heater in the event of an abnormality is also disposed. The safety element is also inserted into another through hole provided in the heater holder.
The sensor and a CPU (control unit) of the device are connected by an electrical cable. The CPU detects the temperature based on the voltage corresponding to the resistance value of the sensor and controls the heater according to the detected temperature.
The temperature detection element and the safety element are disposed so as to be in contact with the heater or in the vicinity of the heater. Therefore, a wire insulation covering a stripped wire of an electrical cable connected to the temperature detection element or the safety element is required to have heat resistance.
Japanese Patent Laid-Open No. 2011-118246 discloses that, by providing a heater holder with protrusions that support an electrical cable, the contact area between the electrical cable and the heater holder is reduced, and heat transfer from the heater to the electrical cable is suppressed.
As a signal line connected to a terminal of the temperature detection element and a power supply line connected to a terminal of the safety element, electrical cables that have a wire insulation as shown in Japanese Patent Laid-Open No. 2011-118246 is used. Since the electrical cables are disposed in the internal space of the film, the electrical cables are required to have not only insulation property but also heat resistance. When the control target temperature of the heater becomes higher with an increase in print speed, it is necessary to use electrical cables having better heat resistance and insulation property.
However, electrical cables that meet these requirements are expensive. In addition, electrical cables that satisfy the heat resistance and the insulation property have a thick wire insulation. Therefore, the space occupied by the electrical cables in the internal space of the film becomes large, which hinders downsizing of the fixing device.